Reproduction processes are known wherein positive-working photopolymerizable elements and negative-working photosensitive elements are exposed imagewise through an original to form nontacky and tacky image areas. Positive-working photopolymerizable elements are described in Chu and Cohen, U.S. Pat. No. 3,649,268, and negative-working photosensitive elements are described in Cohen and Fan, U.S. Pat. Nos. 4,174,216 and 4,191,572. The image is developed by toning with a suitable toner which desirably adheres only in the tacky image areas. Excess toner which may be present is removed from the nontacky areas to provide, for example, an image which is a proof of the original or which can be used to transfer the image to another surface. Multilayer proofs such as surprint or overlay proofs can be made as well.
In view of the increasing importance of proof-making in the printing industry and the problems inherent therein, improved toners and applicators for applying these toners are desirable. Some recognized improvements in nonelectroscopic toners are those described in Chu and Manger, U.S. Pat. No. 3,620,726; mixtures using these toners described in Gray U.S. Pat. No. 3,909,282; and the toners of Manger, Fickes and Long described in U.S. Pat. No. 4,215,193. From the early use of pads dipped in toners, improved toner applicators are the subject of Sandner U.S. Pat. No. 4,019,821 (hand operated toning) and Tobias U.S. Pat. No. 4,069,791 (automatic toning).
Toning the tacky image areas of photosensitive elements by application of any of the above described toners has the problem that the background color resulting from the toners is difficult to completely remove from the nontacky areas of the elements. Generally an attempt is made to mechanically remove excess toner from the nontacky areas of the elements. Generally an attempt is made to mechanically remove excess toner from the nontacky areas using a cloth, brush or other toner removal means. It is known that static electricity is generated by rubbing dissimilar materials together. The amount of static formed varies, depending on the nature of the material being toned and the toner removal means. For example, cleaning brushes which are close to the element in the triboelectric table would be expected to generate a relatively low charge. It would be expected, therefore, that an acrylic brush would generate only a small charge with respect to an element that contains an acrylic compound in its photosensitive layer. The toner, however, can also generate a charge when rubbed against the brush or other cleaning means and the photosensitive element. If the toner, the cleaning means, and the element had an identical place in the triboelectric table, substantially no static charge would be generated. Such a system, while desirable, is not generally achieved. Nevertheless, removal of background color in the nontacky areas is desirable. Manual cleaning is time consuming, and machine cleaning adds to overall equipment costs. Even after clean-up, some undesirable stain is usually present in the nontacky areas.
The term "background color" as used herein is the color present in the nontacky background areas of an exposed and toned positive-working or negative-working photosensitive element before any step is taken to remove such toner. Background color is the sum of two components: "stain", which as used herein is the color which is normally not capable of being removed from the nontacky background areas despite repeated wiping, and "clean-up", which as used herein is the color which normally can be removed from the nontacky background areas, e.g., by wiping, air impingement, etc.
Dry nonelectroscopic toners comprising pigmented organic resin particles have been developed which provide relatively good clean-up and are substantially nonstaining in the nontacky areas of the photosensitive elements used to prepare multilayer proofs. For example, the dry nonelectroscopic toner particles described in Fickes, U.S. Pat. No. 4,397,941, have been surface treated with at least 0.5% by weight of a slip agent, e.g., silicone oil having a weight average molecular weight of about 230 to 50,000; saturated hydrocarbons having a weight average molecular weight of about 200 to 10,000; or fluorocarbon compounds having a weight average molecular weight of about 500 to 500,000, in combination with at least 1% by weight of a defined antistatic agent. Unfortunately, fluorescent pigments are particularly sensitive to various surface coatings, and although the problem of "background color" may be substantially reduced by coating fluorescent pigments as described in the prior art, a rapid and severe deterioration of toning quality makes such treated toners impractical for ordinary use. For example, fluorescent toners coated with a combination of nonionic antistatic agent and silicone oil exhibit reasonable clean-up and stain characteristics when freshly made, but quickly form hard aggregates that render the toners unusable. It is believed that solubility of the coating compounds in the pigmented resin particles may be the cause of these severe aging problems.
Fluorescent pigments have the unusual property of absorbing light at particular frequencies and reemitting this energy at lower frequencies, i.e., longer wavelengths. Many minerals and certain natural products, as well as many synthetic organic compounds and inorganic pigments fluroesce in response to UV radiation. Particularly useful for most toning purposes are pigments that fluroesce in response to both UV radiation and visible light. In daylight fluorescent pigments, the emitted light combines additively with the normal reflected light so that these pigments appear to glow in normal daylight. These intensely vivid colors now find commercial use in a variety of applications, including safety markers, fabrics, signs, plastics and printing inks.
Fluroescent pigments are also used to provide special effects in the photosensitive reproduction processes described above. In this application, however, fluroescent pigments have presented severe problems with respect to background color, i.e., clean-up and stain. These problems are particularly difficult to solve because fluroescent toners surface coated as described in the prior art quickly become unusable due to the formation of hard aggregates.
It is desired to provide dry, nonelectroscopic fluroescent toner particles which when surface coated exhibit improved clean-up qualities and nonstaining of the nontacky areas of photsensitive elements, as well as having excellent toning properties, both initially and after storage. It is particularly desired to avoid the formation of hard aggregates that render such treated toners unusable.